1. Field of the Invention
The present invention relates to a method and apparatus for measuring the methane concentration in a gas, which are particularly useful for measuring the methane concentration in a liquified natural gas (LNG) tanker, an LNG tank or a pit of a coal mine at a distant point.
2. Description of the Prior Art
Methane gas is very important as a fuel gas and is contained in a large quantity in natural gas. With recent increase of the calorific value of the city gas, natural gas has been taking the place of coal gas. Accordingly, in order to prevent occurrence of a gas explosion due to leakage of the city gas, it is eagerly desired to develop a safety system for detecting leakage of a methane gas promptly and assuredly in an underground market, a multi-storied building or the like and giving an alarm.
Methane gas is a main component of a coal mine gas produced in a coal mine, and development of a similar system is desired for preventing an explosion of the coal mine gas and a subsequent explosion of coal dust.
Conventional methane gas sensors of the contact burning type, heat conduction type and semiconductor type are insufficient in the gas selectivity and response characteristics because of the measurement principles of these sensors, and the reliability is readily influenced by the co-present gas, temperature and humidity. Accordingly, these conventional methane gas sensors are not suitable for the measurement in a stope where the measuring conditions are strict, and the real time measurement is difficult. Furthermore, in case of remote inspection and remote measurement, since electric signals are transmitted, a risk of occurrence of an error due to electromagnetic induction or an accident due to damage of a cable cannot be neglected.